In the drilling of deep wells, such as oil wells, a wide variety of earth formations may be encountered at various depths. In many types of earth formations, the formation will be sufficiently consolidated so at to be substantially impermeable to typical drilling mud systems. In some formations, however, it is possible for a substantial quantity of the drilling mud being circulated into the well bore to be lost into the formation. Losses of drilling fluid may be gradual where drilling fluid moves slowly into the formation, or circulation of drilling fluid may be lost entirely when large quantities of the drilling fluid escape into the formation from the well bore.
In the well drilling industry, formations, where partial or total lost circulation occurs, may be classified into four general classes. In one case, the formation may be unconsolidated or highly permeable, such as in the case of loose gravels, for example. In this case, the drilling fluid is slowly injected into the unconsolidated formation by the pressure of the mud system. Since drilling mud may be continually lost in unconsolidated formations, it must be continously replaced by adding more drilling mud to the system. It may be necessary, therefore, to increase the quantity of drilling mud that is ordinarily required for a typical well drilling operation, and this of course is highly undesirable and adversely affects the commercial nature of such well drilling operations.
Some formations have natural fractures that allow substantial quantities of the drilling fluid to flow into the fractures and become lost. It is desirable to seal these fractures and such sealing is conveniently accomplished by adding lost circulation material to the drilling medium so that the lost circulation material will bridge the fracture and provide a seal against further loss of circulation. In some cases, the pressure utilized for the drilling mud system causes natural fractures to widen, thereby compounding the problem of lost circulation.
In some cases, the earth formation is relatively soft and has sufficiently low structural integrity that the pressure of the drilling fluid system can cause the formation to fracture. When this occurs, it is of course necessary to develop a bridging seal at the fracture and provide a formation seal in the same manner as in the case of natural fractures.
In other cases, the earth formation may be referred to as a cavernous formation which may contain naturally occurring openings such as crevices and channels. When such anomalies are encountered during drilling operations, depending upon the particular size and quantity of the openings in the cavernous formation, circulation may be lost completely. The openings or channels may be of sufficient size that extremely large quantities of drilling mud may be lost if drilling is continued. Lost circulation material may be introduced into the drilling mud and may be injected into the well bore either in a slug or by continuous circulation. The lost circulation material enters the structural flaw or abnormality in the formation and causes a formation seal to be developed. The lost circulation material or bridging agents, as sometimes referred to, typically comprise a high filter loss slurry including bridging material that is typically in flake form. Cellophane material in flake or strip form has been utilized successfully as a bridging agent and walnut or pecan shells have also been employed. Many other fibrous or flake materials have also been successfully utilized as bridging agents.
When a condition of lost circulation occurs, drilling mud containing lost circulation material may be continuously circulated through the well bore, thereby causing substantial quantities of the lost circulation material to enter any abnormality in the formation along with drilling mud that is forced into the abnormality. This lost circulation material functions to bridge and seal the abnormality and limits or restricts further loss of drilling fluid into the abnormality. Where the formation is of relatively low structural integrity or is unconsolidated or highly permeable, it is not unusual for the circulating drilling mud to cause substantial erosion or sloughing of the formation. When this occurs, the drilling mud becomes laden with undesirable particulate and it is desirable that the particulate be removed prior to reinjection of the drilling mud into the well bore. It is also desirable, however, to retain the lost circulation material in the drilling mud system during continuous circulation. Screening the drilling mud in the usual manner for removal of undesired particulate matter will also cause removal of the lost circulation material. Such screening will therefore require continuous introduction of new lost circulation material to the drilling mud downstream of the mud screening operation. Correction of lost circulation conditions, when mud screening operations are also necessary or desirable, obviously result in very high drilling costs.
It is therefore a primary feature of the present invention to provide a novel method for processing drilling mud containing lost circulation material such that any undesirable particulate such as drill cuttings or sloughing material will be effectively separated from the drilling mud while at the same time the lost circulation material will be continuously circulated along with the drilling mud.
It is also a feature of the present invention to provide a novel system for processing drilling mud containing lost circulation material which allows continuous circulation of mud and lost circulation material without consequent depletion or use of substantial quantities of the lost circulation material.
It is another important feature of the present invention to provide a novel system for processing drilling mud and lost circulation material, whereby the lost circulation material is separated from the drilling mud immediately upon exit of the drilling mud from the well bore. The drilling fluid is cleaned of undesirable particulate and the lost circulation material is reintroduced into the mud system prior to reinjection of the drilling mud and lost circulation material back into the well bore.
It is a further feature of the present invention to provide a method of processing drilling mud and lost circulation material, whereby the lost circulation material bypasses a particulate separation system in its circulation along with the drilling mud relative to the well bore being drilled.
Among the several features of the present invention is noted the contemplation of novel mechanism for processing drilling mud containing lost circulation material, which mechanism continuously and automatically accomplishes separation of the lost circulation material exiting from the well bore, while at the same time cleaning the drilling mud of undesired particulate matter and reintroducing the previously separated lost circulation material into the cleaned drilling mud prior to reinjection of the drilling mud/lost circulation material into the well bore.
The present invention has as another important feature, the advantage of allowing continuous circulation of drilling mud and lost circulation material and continuous cleaning of particulate from the drilling mud without allowing substantial depletion of either the drilling mud or the lost circulation material, thereby accomplishing efficient and low cost well drilling operations and at the same time efficiently combating the problems of lost circulation and losses of drilling mud into the formation.
Other and further features and advantages of the present invention will become apparent to those skilled in the art upon consideration of the present disclosure. The form of the invention, which will now be described in detail illustrates the general principles of the invention, but it is to understood that this detailed description is not to be taken as limiting the scope of the present invention.